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89 TBI 350 stumbling around cruise

I am having a stumbling problem with my 1989 Chevy S10, which I have retrofitted a 350 smallblock into.

I more or less followed the JTR book in doing this. Temperature control hasn't been a problem, so doubt that is the issue.

Symptoms are, at idle and moderate cruise, it has a bit of a stumble. I haven't been able to get a great grasp upon the particulars of when it's doing it, except to say it doesn't appear to be at heavy throttle. Sitting at idle at a light, I can feel the truck shake now and then, I assume this is a misfire. Occasionally it will cut out- sometimes at idle, sometimes while cruising, though that is quite rare. Also, it will occasionally backfire through the exhaust, but I think that is to be expected with a misfire/stumble. It only does it maybe twice a week.

I am using the correct ECU. I am using the correct injectors (in fact they are about a year old). Pattern is fine, though I can't say they aren't delivering too much fuel.

-Fuel pump is new, TPI unit as advised by a member here to ensure fueling for future mods (TPI pump puts out in the neighborhood of 50PSI, TBI needs about 15, as I'm sure you guys know). I assume the FPR is doing its job and regulating pressure down.
-FPR is new, replaced when I pulled the TB and installed new injectors.
-Distributor is new unit from Oreilly. (well, 3 years old. New enough).
-Ignition module is new.
-Coil is new.
-Coil harness is new (connectors were broken so I replaced the 2-wire connection to the ign module as well as both connections to the coil. This actually improved the situation for a while, but the problem is creeping back.
-Coil has been relocated to the firewall, due to interference from the oil pressure switch (and tap for bypass filter).
-IAC is new.
-Plugs are new (Autolite, no "trick pony" plugs here)
-Plug Wires are new, and very protected (due to manifolds and packaging, I had to make my own wires. Used Accel ratchet crimper, Accel 8mm wire, and jacketed them in antichafe/heat protective fiberglass/silicone material. Ran them a la 76 Vette- down behind the heads, to the oil pan, and forward along the oil pan rails, held by Addel clamps. Pretty darn confident I am not having a "chafe and ground to the block" problem here.)

My thoughts:
-Need to verify TPS range, and replace if questionable.
-Need to refresh grounds- been having electrical gremlins for a while- gauges acting up and the like. Do the "Big 3" and grounds. I have buss bars to do this, and am awaiting copper lugs to finish the job.
-TBI could use a good scrubbing. In fact, I have a second unit, might be time to go ahead and scrub it really well and modify for JIC fittings, in order to fab new fuel lines and install a gauge at the TB unit itself.
-My exhaust is slightly "holy". As in, the exhaust crossover pipe was made from the old 4.3 Y-pipe, cut and welded with new pipe. It's not my welds that are the problem, but the old pipe- had some noticeable pinholes. This would be upstream of the o2 sensor, which of course would probably play hell with it. I know it was wrong when I did it, but sometimes you have to do what you have to do. On that front, I am considering running duals down the passenger side (due to lift and mods, I can stand up two mufflers side-by-side and package them in the place of the old). In this case, I will run 2.25" pipe front to back, and I'll have a connection between the two - h-pipe - in the vicinity of the junction of the old Y pipe.

I ran Seafoam through the last tank. I need to do the heavy cleaning protocol with it, though- pour it down the TBI till it chokes the engine, then let it sit. I may have an issue here with carbon in the motor. I doubt it, and if need be I can always inspect via a borecam.

I just checked for engine codes with the old "paperclip on ALDL A and B terms" trick. Nothing noted- just 12 over and over.

I am going to go ahead and fabricate new positive cables from battery to starter, and battery to alternator, simply because the OEM stuff is looking rather ratty, and I also added a fan assembly with 75-Amp inrush, 40-Amp steady ratings. Granted, it's actuated by relays and runs off a piece of 8 gauge to the battery, but the battery (and thus the rest of the truck) is still supplied by a suspect looking piece of 10 gauge. At any rate it absolutely will not hurt.
I am also going to make new grounds- firewall/body to block, block to bussbar, frame to bussbar, bussbar to battery. I will be making these of 2-GA welding cable, hammercrimped and soldered with marine grade heatshrink to seal them. I don't expect this to be a magic bullet, but I do think it will be part of it.

The reason the parts were replaced is- to get the engine running. It came to me without a distributor, and I pulled the TB off the 4.3 and put it on the 5.7 (and to recap, yes I did install the proper injectors, and I also replaced all the gaskets and FPR in the vicinity while I did it. The distributor was broken on the new motor (motor was running strong in the junkyard truck I drove it in, I damaged the dizzy in lashing down the motor for transpo).

Fuel pump was replaced as it was suspect after I've driven the truck for 20 years without touching it, and the vehicle was deadlined at the time. I used a Delphi unit, as it has a good reputation for quality. I get your point about even new parts being possibly bad, though.

Ignition module was tested at the parts store on their tester. At the time it was suspect (when I started having this problem). I tested it 10 times, it failed the last 8 (always warm up parts when testing out of position, as heat is the enemy of electronics). It failed High-RPM output and SPOUT each time. It was a warranty replacement.

Coil was replaced, as I'd been running an old 1992-vintage Accel Supercoil. It had seen service in a couple Jeeps, a couple Fieros, and the Blazer for the last 20 years. It was questionable at the time according to my readings (which I now question, as the meter smoked on me shortly thereafter) so I splurged the $15 for a new OEM one. Coil may still be the issue, if my grounds are aas questionable as I suspect- I have yet to run a ground strap from it's new home to the block. That will change soonest. Tonight, if it's not raining.

Plug wires were chafing to the core, so replaced with fabricated units. I've rung them out good (IIRC, 50 ohms) but will re-ring at some point, likely the weekend. St the same time I'll read the plugs.

You said "datalog for trims "... might as well be speaking Greek to me. I'm VERY new to this- if I log the data can I upload it so you can look over it and advise? I don't expect you to spoonfeed me, but a nudge in the right direction would be much appreciated.

EDIT to add: the coil harness is new- I fabricated it myself with connectors bought at Oreilly. That's nothing meant to impress- it's simple work. But I did twist the wires from the ignition module 2-wire harness to the coil (prevents crosstalk and inductive-coupling, and doesn't cause issues). Everything was soldered and heatshrunk. Yes I used electrical solder, not plumbing- the difference being mainly in the rosin flux instead of acid flux (I've done lots of soldering work on DoD equipment and high-end autosound and security work, so my connections aren't the problem, guarantee 100%). I taped and loomed the new harness, and it was built long enough to where it's not tugging away from the connections- in short, there is no way the harness I built is the problem).

get some plastic pliers and disconnect one plug wire at a time at idle, make sure it's running on all cylinders evenly

I offer a suggestion that will function for cylinder RPM drop that will not shock you. I've had these plastic pliers conduct and shock me.

i have a huge amount of trouble imagining spark voltage conducting itself through nearly 6" of dry plastic, especially after having to jump through a spark plug boot. and if it did, it should suffer such resistance that it'd be like touching a 9v battery.

i've use them tons of times, maybe over a thousand plug wires by now, never a tickle

I've been shocked using plastic pliers also. Maybe it's just the universe taking advantage of the opportunity to mess with me. I've been shocked so damn many times it's not even funny anymore.

Well, I should say it's not funny to me. There's no end to the amusement I generate for spectators.

I can't see what suggestion was posted but I often use short lengths of vacuum tubing between plug wire and coil then use a test light from ground to the vacuum hose. I am also likely to use a grounded test light at the end of the spark plug boot if I choose to disconnect the plug wire while the engine is running.

I've been shocked using plastic pliers also. Maybe it's just the universe taking advantage of the opportunity to mess with me. I've been shocked so damn many times it's not even funny anymore.

Well, I should say it's not funny to me. There's no end to the amusement I generate for spectators.

I can't see what suggestion was posted but I often use short lengths of vacuum tubing between plug wire and coil then use a test light from ground to the vacuum hose. I am also likely to use a grounded test light at the end of the spark plug boot if I choose to disconnect the plug wire while the engine is running.

I fought that problem for years in my truck. It turned out to be both heads were cracked at the exhaust seats. And after it warmed up it would miss/ stumble at idle and light throttle. 193 tmi heads are prone to crack at the exhaust seats. My 2 cents

I fought that problem for years in my truck. It turned out to be both heads were cracked at the exhaust seats. And after it warmed up it would miss/ stumble at idle and light throttle. 193 tmi heads are prone to crack at the exhaust seats. My 2 cents

Are you referring to my problem, it's not clear given the quote above your post.

I haven't been able to install the new grounds yet (owing to work and weather), but I have fabricated a few of them. One thing I did notice is when I relocated the sensor grounds from the thermostat bolt to the manifold bolt nearby, apparently I didn't run a ground from there off to the battery/body. So that is a HIGHLY suspect potential cause of this issue. Some would say relocating the grounds form the thermostat bolt is the issue you'll just never convince me a bolt threaded into an aluminum manifold is any better a ground than using a bolt going into the iron head (which, if we are being picky, the manifold was bolted into, so it should actually be a BETTER ground as it cuts down on the chain of grounds to get back to the negative terminal on the battery. ... well, you know, if I'd actually run that darn wire ;) )

This is clearly going to be the FIRST ground I improve. I built a 2-GA cable out of welding cable with a copper lug hammer-crimped to it and soldered (honestly, solder would be just fine on its own, and I've built many a cable that way) with marine-grade heatshrink over 3M electrical tape to seal against the weather.

Are you referring to my problem, it's not clear given the quote above your post.

I haven't been able to install the new grounds yet (owing to work and weather), but I have fabricated a few of them. One thing I did notice is when I relocated the sensor grounds from the thermostat bolt to the manifold bolt nearby, apparently I didn't run a ground from there off to the battery/body. So that is a HIGHLY suspect potential cause of this issue. Some would say relocating the grounds form the thermostat bolt is the issue you'll just never convince me a bolt threaded into an aluminum manifold is any better a ground than using a bolt going into the iron head (which, if we are being picky, the manifold was bolted into, so it should actually be a BETTER ground as it cuts down on the chain of grounds to get back to the negative terminal on the battery. ... well, you know, if I'd actually run that darn wire ;) )

This is clearly going to be the FIRST ground I improve. I built a 2-GA cable out of welding cable with a copper lug hammer-crimped to it and soldered (honestly, solder would be just fine on its own, and I've built many a cable that way) with marine-grade heatshrink over 3M electrical tape to seal against the weather.

Doing this on my phone, only gives me the option of reply with quote. I also replaced all the grounds and power with 2 GA wire fixed my low voltage problems, but not the stumble. I only found the problem after a backfire caused a small fire in the engine compartment and I decided to tear down the motor and freshen it up. Both heads went to the machine shop for a valve job, and both were cracked. Not say that is your problem, but it is a possibility

Well, I apparently had replaced the grounds in the past, with 10GA to each head, to a stud on the passenger fenderwell used as a ground terminal (yes, very hack, I agree, but all I had at the time). I added 2GA grounds to each head, just forward of the intake. ran them to a buss bar, found here: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Ran a jumper from the battery to the buss bar as well. Ran another from the firewall under the coil bolt, to the driver's side (also where all the sensors get their ground).

Added a red version of the above, to the driver's side firewall. Ran all my aux circuits to it, and it's fed by a piece of 2GA. Replaced the alternator wire (factory, approx 10GA) with a piece of 2GA. All terminals are hammer-crimped and soldered, with marine heatshrink over them.

It's too soon to tell, but she sure seemed to run better this morning. I doubt this was all of the problem, but it may have been some.

I also note a new development on my gauges. The oil pressure gauge used to read fine on cold start, then go to nothing on hot idle. I'd verifed with a mechanical gauge that it was fine so I wasn't too excited. Now, with key on and not running, it shows pegged at 60PSI. running, same thing. Drops to nil at key off (as it should). I think it's time to pull the panel and check my grounds on it. I likely have some issues there, at the very least I need to replace bulbs. That may well be part of the issue, actually- absent groundplane due to the bulbs being blown. I have LEDs to replace them, just haven't had time to suss it out.

My neighbor has a 97 Tacoma he wants to sell, for $2000. The truck books out somewhere around 4G. Done! 22MPG with cold air, and a parts runner (bed) so I can get some of the unwiledly crap I've been needing to get (engine for the jeep) as well as household stuff I need to run to the dump taken care of. It's a no-brainer.

Also alot of people just dont think of this but as part of the tune up its a good idea to pull off the tbi unit and rebuild it, the o rings shrink, especially of they havent been replaced since they started using ethonol in the gas,and the pod half gaskets get old and leak and regulators go bad and leak excess fuel. Not sure how long you have had the car, but I pull off and rebuild em on every vehicle I buy with tbi, over the years other so call mecahanics get in there and really screw shit up, I bought a 92 4.3 4x4 s10 and when i pulled the tbi unit off, it had no base gasket, then the pod housing was covered in jb weld because the idiots installed the injectors in backwards and it wouldnt sit down flush because the pins where not in the notches..LOL I tell you man Ive seen it all.

Pretty sure I mentioned I'd rebuilt the TBI unit. I may be mistaken. At any rate, I agree with you, and have done it.

I have another TBI unit that I will mod for JIC connections, a Fuel Pressure Gauge right at the back of the unit, etc. Of course I will tank it and rebuild it thoroughly while I'm at it.

As I purchased that Toyota I referred to in the past, I can now afford to take the Blazer down and do it up right. One of the big things I need to do is build new axles- the gearing is all wrong in these.